Offset printing is a printing system utilizing the water repelling property of an oil-based printing ink composition and using a printing plate not having asperities, unlike a relief printing system using a printing plate having asperities. This printing plate has lipophilic image areas and hydrophilic non-image areas, instead of asperities. First, the non-image areas are first wetted with a dampening solution supplied to the printing plate for printing. Second, an oil-based offset printing ink composition (hereinafter, also simply referred to as printing ink composition) is supplied to the printing plate. The non-image areas charged with water by the application of the dampening solution repel the printing ink composition and prevent the printing ink composition from adhering. In other words, the printing ink composition adheres to only the lipophilic image areas. An image of the printing ink composition is thereby formed on the printing plate, and then is transferred to a blanket and paper to complete printing.
Instead of the offset printing using a dampening solution as described above, a water-free offset printing system using a printing plate having non-image areas formed with a silicone resin has been put into practical use. In this printing system, the silicone resin, not a dampening solution, repels the printing ink composition to form non-image areas. Except in this respect, the water-free offset printing is identical to the offset printing using a dampening solution. In this specification, the term “offset printing” includes both printing systems using dampening solution and water-free printing systems.
Incidentally, if the printing ink composition adhering to the surface of a printed sheet prepared by offset printing is not sufficiently dried, the ink composition of the printed sheet will transfer to the back surface of another printed sheet laid on top of the printed sheet, or the ink will adhere to a finger in contact with the printed sheet. Accordingly, the printed sheet cannot be sent to any post-processing or be distributed as a product. A process for drying the printing ink composition adhering to the surface of a printed sheet is therefore required after the offset printing. The drying systems of the printing ink composition in the offset printing system are categorized into the following four types: oxidative polymerization, evaporation, infiltration, and photopolymerization, which are selected depending on the type of the printing ink composition used.
Among these systems, the system of drying a printing ink composition by infiltration dries the surface of a printed sheet by allowing the oil component contained in the printing ink composition adhering to the surface to infiltrate into the inside of the printed sheet and can achieve a dried state relatively quickly. In this drying system, however, the printing ink composition readily spreads on the surface of the printed sheet into enlarged dots. In addition, this drying system is unsuitable for printing on sheets showing slow infiltration of oil components, such as glossy paper, and is unsuitable for application to a field requiring high quality aesthetics, such as product packages and catalogs. The system of drying a printing ink composition by evaporation could cause environmental pollution by releasing a volatile organic compound (VOC) into the atmosphere, and is therefore rarely used.
From such a viewpoint, in order to prepare a printed sheet that is required to have high quality aesthetics, current trend is to use a system of drying a printing ink composition by oxidative polymerization. The printing ink composition used in this drying system contains unsaturated oil having a high iodine value, such as tung oil or linseed oil, and the unsaturated oil is oxidatively polymerized by oxygen in the air to form a non-sticky (i.e., dried) coating film. The system of drying a printing ink composition by oxidative polymerization does not have a process of infiltrating the oil component into the printed sheet or releasing the VOC to the atmosphere and therefore can provide printed matter having high aesthetics with a low environmental burden. Since the chemical reaction of oxidative polymerization of unsaturated oil takes a relatively long time, there is a risk of causing a quality problem due to incomplete drying of the printed sheet.
From the above-described background, printing employing the drying system through photopolymerization, i.e., drying through irradiation of a printed sheet with UV rays after printing has been extensively performed recently. The printing ink composition used in this drying system contains an ethylenically unsaturated monomer or oligomer and a photopolymerization initiator generating a radical by irradiation with active energy rays, such as UV rays, and forms a non-sticky (i.e., dried) coating film via quick polymerization of these components by irradiation with active energy rays after printing. Various offset printing ink compositions employing such a drying system have been proposed recently (for example, see Patent Literatures 1 and 2).